I am a very happy user of the Greek Polytonic Keyboard (and of the Latin Unicode as well), but would be even happer if I had a visual keyboard for each, rather than looking at a printed map as well as the document I'm typing. Do the .kbk files come with the package and I've mislaid them, or do I (a) have to download them separately, or even (b) make them up for myself with Microsoft Keyboard Layout Creator. At my age, I'm not sure I've enough time left for the latter.

A visual keyboard is normally created alongside the keyboard with Keyman Developer. However, no visual keyboard was created for this keyboard, so there isn't one available -- the value of one is debatable as the keyboard uses a lot of modifier keys and some key combinations for diacritics.

There is a keyboard layout diagram in the PDF documentation for that keyboard -- it should be installed along with the keyboards.